1. Field of the Invention
The invention relates to alkylthio- and aryl(heteroyl)thio-substituted p-phenylenediamines, their manufacture and use as antidegradants in rubber compounds.
2. Discussion of the Prior Art
Vulcanizing rubber compositions by heating a sulfur-vulcanizable rubber composition with sulfur and/or a sulfur donor and a vulcanization accelerator has been known for many years. By this process vulcanizates having acceptable physical properties including tensile strength, resilience, and fatigue resistance can be obtained, but such vulcanizates tend not to have good aging properties.
Uncured as well as cured rubbers are prone to aging effects. The unsaturated groups in diene rubbers, e.g. styrene-butadiene rubber (SBR) or a blend of SBR with natural rubber, butadiene rubber or with both, make it possible to cure with sulfur, but at the same time they exhibit a sensitivity toward oxygen, ozone, and other reactive substances causing changes such as hardening of the vulcanizate. Unaged diene rubbers contain free double bonds that remain sensitive to the above reactive substances even after vulcanization. Higher temperatures make these effects even more noticeable.
Protective agents are used to protect the rubber vulcanizate against various forms of aging, fatigue, and ozone. For example, exposure of pneumatic tires to ozone leads to the formation of ozone cracks, in particular in the sidewalls of the tire. A well-known class of protective agents are N,N′-di-substituted, in particular N-alkyl-N′-phenyl-p-phenylenediamine derivatives. These N,N′-di-substituted p-phenylenediamine derivatives typically are also referred to as antidegradants, antiozonants or antioxidants. The reader is directed to Hofmann, Rubber Technology Handbook, Hanser Publishers, Munich 1989, pp. 264-277, in particular pp. 269-270. These antidegradants are commercially available inter alia under the trademark Santoflex® sold by Flexsys America L.P. In the rubber industry, the most frequently used antidegradant is N-(1,3-dimethylbutyl)-N′-phenyl-p-phenylenediamine or 6PPD.
Known 1,4-benzoquinone diimines (QDI's) can be classified under three major categories: I, II and III. Class I compounds contain no substituents attached to the benzoquinoid ring. Class II, the largest group, contains amino-substituents at the 2- and 5-positions of the benzoquinoid ring. Class III comprises all other p-benzoquinone diimines.
Many benzoquinone diimines (QDI's) were previously prepared by the oxidation of phenylenediamine derivatives with manganate, ferricyanate, iodine, silver ion, silver oxide, and lead tetraacetate (or “red lead”).
Oxidation of N,N′-diphenyl-p-phenylenediamine with Ag2O gives N,N′-diphenyl-p-phenylenediimine. Oxidation of N-(1,3-dimethylbutyl)-N′-phenyl-1,4-phenylenediamine (6PPD) an effective antiozonant and antioxidant used in rubber industry, with Ag2O gives the corresponding QDI in 55% yield; similar conversion is achieved by photocatalytic oxidation using Ru3. The QDI can be prepared by two consecutive photo cleavages of NO groups from the bis-nitroso derivative of N,N′-diphenyl-p-phenylenediamine.
N,N′-Diaryl-2,5-bis(arylamino)-p-benzoquinone diimines or azophenines, which belong to the second class of QDI's and are dye intermediates, have been synthesized by a wide variety of methods many of which involve the separation of resulting mixtures and afford low yields. Parent azophenine has been prepared by heating p-benzoquinonedianil with aniline. Substituted azophenines are also formed in 25-35% yields by oxidation of anilines on heating with 1,1,2,2-tetrachloroethane or hexachloroethane in the presence of copper bronze. The oxidation of aniline by 3- and 4-azidopyridine-1-oxides also gives azophenines. Peroxidase oxidation of 4-chloroaniline gives 2-amino-5-chloroanilinobenzoquinone di-4-chloroanil. Benzoquinone diimines are formed as side products in the decomposition of the corresponding nitroxide.
N,N′-Bis[phenylsulfonyl]-1,4-benzoquinone diimine is sulfanylamidated by sodium N-chlorobenzenesulfonamide to form 2,5-bis(phenylsulfonylamino)-N,N′-bis(phenylsulfonyl)-1,4-benzoquinone diimine. Similarly, amidation of N,N′-bis[phenylsulfonyl]-1,4-benzoquinone diimine with N-chloroamides gives the corresponding sulfonyl benzoquinone diimine derivatives in one step.
Few synthetic procedures are known for the preparation of representatives of class III of benzoquinone diimines. For example, many symmetrical and unsymmetrical N,N′-bis(arylthio)- and N,N′-bis(arylseleno)-quinone diimines have been prepared by treatment of N,N′-dichloroquinone diimines with thiols or selenols, respectively.
More recently, N,N′-dicyanoquinone diimine (DCNQI) salts have gained attention due to their high conductivity and ease of synthesis from benzoquinones and bis(trimethylsilyl)carbodiimide.
A synthetic route to poly(quinone diimines) via the treatment of anthraquinone (AQ) with aromatic diamines in the presence of TiCl4 and 1,4-diazabicyclo[2.2.2]octane (Dabco) has been developed.
Employing the same model as above, some heterocyclic quinone arylimines have been prepared through Wittig addition of (N-aryl)triphenylarsinimines to the carbonyl functionality of heterocyclic benzoquinone derivatives. Polyaromatic quinone imines are readily formed by reactions of the corresponding quinones with triphenylarsine oxide and aryl isocyanates.
Most reactions of p-benzoquinone diimines fall into two broad types: reduction and addition. The reactivity of benzoquinone diimines (QDI) is dictated by a strong tendency to form a stable benzenoid structure. Therefore, they are very reactive towards nucleophilic addition and undergo reduction more readily than quinones. Most published reactions of QDI's relate to studies of the relatively stable diacyl and disulfonyl derivatives.
In the publication by Snell and Weissberger, The Reaction of Thiol Compounds with Quinones, JACS 61, 450(1938), the reactions between thiol compounds with benzoquinone and substituted benzoquinones were discussed. The article stated that two types of reaction may be expected: oxidation of the thiol to a disulfide with reduction of the quinone to the hydroquinone; and addition of the thiol to the quinone to obtain alkylthio substituted quinones and/or hydroquinones.
In Gelling and Knight, Rubber chemistry of N-substituted quinone imines and N,N′-disubstituted quinone diimines, Plastics and Rubber Processing September 1977, findings were discussed concerning two distinct reactions that occur between 2-mercaptobenzothiazole (MBT) and N-cyclohexyl-N′-phenylquinone diimine.
The major reaction involves 1,4-addition of the MBT across the diimine ring to yield the addition product. There is also an oxidation-reduction reaction to yield N-cyclohexyl-N′-phenyl-p-phenylenediamine and 2,2′-dithio-bis(benzothiazole).